]>
Commit | Line | Data |
---|---|---|
59c06be3 | 1 | .\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com> |
7a0d1838 PZ |
2 | .\" and Copyright (C) 2014 Peter Zijlstra <peterz@infradead.org> |
3 | .\" and Copyright (C) 2014 Juri Lelli <juri.lelli@gmail.com> | |
59c06be3 MK |
4 | .\" Various pieces from the old sched_setscheduler(2) page |
5 | .\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999 | |
6 | .\" and Copyright (C) 2007 Carsten Emde <Carsten.Emde@osadl.org> | |
7 | .\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com> | |
8 | .\" | |
9 | .\" %%%LICENSE_START(GPLv2+_DOC_FULL) | |
10 | .\" This is free documentation; you can redistribute it and/or | |
11 | .\" modify it under the terms of the GNU General Public License as | |
12 | .\" published by the Free Software Foundation; either version 2 of | |
13 | .\" the License, or (at your option) any later version. | |
14 | .\" | |
15 | .\" The GNU General Public License's references to "object code" | |
16 | .\" and "executables" are to be interpreted as the output of any | |
17 | .\" document formatting or typesetting system, including | |
18 | .\" intermediate and printed output. | |
19 | .\" | |
20 | .\" This manual is distributed in the hope that it will be useful, | |
21 | .\" but WITHOUT ANY WARRANTY; without even the implied warranty of | |
22 | .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
23 | .\" GNU General Public License for more details. | |
24 | .\" | |
25 | .\" You should have received a copy of the GNU General Public | |
26 | .\" License along with this manual; if not, see | |
27 | .\" <http://www.gnu.org/licenses/>. | |
28 | .\" %%%LICENSE_END | |
29 | .\" | |
30 | .\" Worth looking at: http://rt.wiki.kernel.org/index.php | |
31 | .\" | |
b8efb414 | 32 | .TH SCHED 7 2016-10-08 "Linux" "Linux Programmer's Manual" |
59c06be3 | 33 | .SH NAME |
cfd62fa2 | 34 | sched \- overview of CPU scheduling |
59c06be3 | 35 | .SH DESCRIPTION |
30af6b5d MK |
36 | Since Linux 2.6.23, the default scheduler is CFS, |
37 | the "Completely Fair Scheduler". | |
38 | The CFS scheduler replaced the earlier "O(1)" scheduler. | |
39 | .\" | |
b16695a3 | 40 | .SS API summary |
bac6ef74 MK |
41 | Linux provides the following system calls for controlling |
42 | the CPU scheduling behavior, policy, and priority of processes | |
43 | (or, more precisely, threads). | |
b16695a3 | 44 | .TP |
31046c3c MK |
45 | .BR nice (2) |
46 | Set a new nice value for the calling thread, | |
47 | and return the new nice value. | |
48 | .TP | |
49 | .BR getpriority (2) | |
50 | Return the nice value of a thread, a process group, | |
51 | or the set of threads owned by a specified user. | |
52 | .TP | |
53 | .BR setpriority (2) | |
54 | Set the nice value of a thread, a process group, | |
55 | or the set of threads owned by a specified user. | |
56 | .TP | |
b16695a3 MK |
57 | .BR sched_setscheduler (2) |
58 | Set the scheduling policy and parameters of a specified thread. | |
59 | .TP | |
60 | .BR sched_getscheduler (2) | |
61 | Return the scheduling policy of a specified thread. | |
62 | .TP | |
63 | .BR sched_setparam (2) | |
64 | Set the scheduling parameters of a specified thread. | |
65 | .TP | |
66 | .BR sched_getparam (2) | |
67 | Fetch the scheduling parameters of a specified thread. | |
68 | .TP | |
69 | .BR sched_get_priority_max (2) | |
275e3c16 | 70 | Return the maximum priority available in a specified scheduling policy. |
b16695a3 MK |
71 | .TP |
72 | .BR sched_get_priority_min (2) | |
275e3c16 | 73 | Return the minimum priority available in a specified scheduling policy. |
b16695a3 | 74 | .TP |
5813ff92 | 75 | .BR sched_rr_get_interval (2) |
b16695a3 MK |
76 | Fetch the quantum used for threads that are scheduled under |
77 | the "round-robin" scheduling policy. | |
78 | .TP | |
79 | .BR sched_yield (2) | |
80 | Cause the caller to relinquish the CPU, | |
81 | so that some other thread be executed. | |
82 | .TP | |
83 | .BR sched_setaffinity (2) | |
84 | (Linux-specific) | |
85 | Set the CPU affinity of a specified thread. | |
86 | .TP | |
87 | .BR sched_getaffinity (2) | |
88 | (Linux-specific) | |
91f5e870 | 89 | Get the CPU affinity of a specified thread. |
b16695a3 MK |
90 | .TP |
91 | .BR sched_setattr (2) | |
77dab50a MK |
92 | Set the scheduling policy and parameters of a specified thread. |
93 | This (Linux-specific) system call provides a superset of the functionality of | |
94 | .BR sched_setscheduler (2) | |
95 | and | |
96 | .BR sched_setparam (2). | |
b16695a3 MK |
97 | .TP |
98 | .BR sched_getattr (2) | |
77dab50a MK |
99 | Fetch the scheduling policy and parameters of a specified thread. |
100 | This (Linux-specific) system call provides a superset of the functionality of | |
101 | .BR sched_getscheduler (2) | |
102 | and | |
103 | .BR sched_getparam (2). | |
b16695a3 | 104 | .\" |
59c06be3 MK |
105 | .SS Scheduling policies |
106 | The scheduler is the kernel component that decides which runnable thread | |
107 | will be executed by the CPU next. | |
108 | Each thread has an associated scheduling policy and a \fIstatic\fP | |
2e868ccc MK |
109 | scheduling priority, |
110 | .IR sched_priority . | |
961df2a8 | 111 | The scheduler makes its decisions based on knowledge of the scheduling |
59c06be3 MK |
112 | policy and static priority of all threads on the system. |
113 | ||
114 | For threads scheduled under one of the normal scheduling policies | |
115 | (\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP), | |
116 | \fIsched_priority\fP is not used in scheduling | |
117 | decisions (it must be specified as 0). | |
118 | ||
119 | Processes scheduled under one of the real-time policies | |
120 | (\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a | |
121 | \fIsched_priority\fP value in the range 1 (low) to 99 (high). | |
122 | (As the numbers imply, real-time threads always have higher priority | |
123 | than normal threads.) | |
cc401eea | 124 | Note well: POSIX.1 requires an implementation to support only a |
59c06be3 MK |
125 | minimum 32 distinct priority levels for the real-time policies, |
126 | and some systems supply just this minimum. | |
127 | Portable programs should use | |
128 | .BR sched_get_priority_min (2) | |
129 | and | |
130 | .BR sched_get_priority_max (2) | |
131 | to find the range of priorities supported for a particular policy. | |
132 | ||
133 | Conceptually, the scheduler maintains a list of runnable | |
134 | threads for each possible \fIsched_priority\fP value. | |
135 | In order to determine which thread runs next, the scheduler looks for | |
136 | the nonempty list with the highest static priority and selects the | |
137 | thread at the head of this list. | |
138 | ||
139 | A thread's scheduling policy determines | |
140 | where it will be inserted into the list of threads | |
141 | with equal static priority and how it will move inside this list. | |
142 | ||
143 | All scheduling is preemptive: if a thread with a higher static | |
144 | priority becomes ready to run, the currently running thread | |
145 | will be preempted and | |
146 | returned to the wait list for its static priority level. | |
147 | The scheduling policy determines the | |
148 | ordering only within the list of runnable threads with equal static | |
149 | priority. | |
150 | .SS SCHED_FIFO: First in-first out scheduling | |
151 | \fBSCHED_FIFO\fP can be used only with static priorities higher than | |
152 | 0, which means that when a \fBSCHED_FIFO\fP threads becomes runnable, | |
153 | it will always immediately preempt any currently running | |
154 | \fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP thread. | |
155 | \fBSCHED_FIFO\fP is a simple scheduling | |
156 | algorithm without time slicing. | |
157 | For threads scheduled under the | |
158 | \fBSCHED_FIFO\fP policy, the following rules apply: | |
159 | .IP * 3 | |
160 | A \fBSCHED_FIFO\fP thread that has been preempted by another thread of | |
161 | higher priority will stay at the head of the list for its priority and | |
162 | will resume execution as soon as all threads of higher priority are | |
163 | blocked again. | |
164 | .IP * | |
165 | When a \fBSCHED_FIFO\fP thread becomes runnable, it | |
166 | will be inserted at the end of the list for its priority. | |
167 | .IP * | |
168 | A call to | |
4c2eb0c2 MK |
169 | .BR sched_setscheduler (2), |
170 | .BR sched_setparam (2), | |
59c06be3 | 171 | or |
4c2eb0c2 | 172 | .BR sched_setattr (2) |
59c06be3 MK |
173 | will put the |
174 | \fBSCHED_FIFO\fP (or \fBSCHED_RR\fP) thread identified by | |
175 | \fIpid\fP at the start of the list if it was runnable. | |
176 | As a consequence, it may preempt the currently running thread if | |
177 | it has the same priority. | |
cc401eea | 178 | (POSIX.1 specifies that the thread should go to the end |
59c06be3 MK |
179 | of the list.) |
180 | .\" In 2.2.x and 2.4.x, the thread is placed at the front of the queue | |
181 | .\" In 2.0.x, the Right Thing happened: the thread went to the back -- MTK | |
182 | .IP * | |
183 | A thread calling | |
184 | .BR sched_yield (2) | |
185 | will be put at the end of the list. | |
186 | .PP | |
187 | No other events will move a thread | |
188 | scheduled under the \fBSCHED_FIFO\fP policy in the wait list of | |
189 | runnable threads with equal static priority. | |
190 | ||
191 | A \fBSCHED_FIFO\fP | |
192 | thread runs until either it is blocked by an I/O request, it is | |
193 | preempted by a higher priority thread, or it calls | |
194 | .BR sched_yield (2). | |
195 | .SS SCHED_RR: Round-robin scheduling | |
196 | \fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP. | |
197 | Everything | |
198 | described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP, | |
199 | except that each thread is allowed to run only for a maximum time | |
200 | quantum. | |
201 | If a \fBSCHED_RR\fP thread has been running for a time | |
202 | period equal to or longer than the time quantum, it will be put at the | |
203 | end of the list for its priority. | |
204 | A \fBSCHED_RR\fP thread that has | |
205 | been preempted by a higher priority thread and subsequently resumes | |
206 | execution as a running thread will complete the unexpired portion of | |
207 | its round-robin time quantum. | |
208 | The length of the time quantum can be | |
209 | retrieved using | |
210 | .BR sched_rr_get_interval (2). | |
211 | .\" On Linux 2.4, the length of the RR interval is influenced | |
212 | .\" by the process nice value -- MTK | |
213 | .\" | |
7a0d1838 | 214 | .SS SCHED_DEADLINE: Sporadic task model deadline scheduling |
9cc1fa25 MK |
215 | Since version 3.14, Linux provides a deadline scheduling policy |
216 | .RB ( SCHED_DEADLINE ). | |
217 | This policy is currently implemented using | |
218 | GEDF (Global Earliest Deadline First) | |
219 | in conjunction with CBS (Constant Bandwidth Server). | |
220 | To set and fetch this policy and associated attributes, | |
221 | one must use the Linux-specific | |
222 | .BR sched_setattr (2) | |
223 | and | |
224 | .BR sched_getattr (2) | |
225 | system calls. | |
7a0d1838 PZ |
226 | |
227 | A sporadic task is one that has a sequence of jobs, where each | |
91c98da6 | 228 | job is activated at most once per period. |
9cc1fa25 MK |
229 | Each job also has a |
230 | .IR "relative deadline" , | |
91c98da6 | 231 | before which it should finish execution, and a |
9cc1fa25 MK |
232 | .IR "computation time" , |
233 | which is the CPU time necessary for executing the job. | |
234 | The moment when a task wakes up | |
235 | because a new job has to be executed is called the | |
236 | .IR "arrival time" | |
237 | (also referred to as the request time or release time). | |
238 | The | |
239 | .IR "start time" | |
240 | is the time at which a task starts its execution. | |
241 | The | |
0da5e58a | 242 | .I "absolute deadline" |
9cc1fa25 | 243 | is thus obtained by adding the relative deadline to the arrival time. |
7a0d1838 PZ |
244 | |
245 | The following diagram clarifies these terms: | |
246 | ||
91c98da6 | 247 | .in +4n |
7a0d1838 | 248 | .nf |
7bd7f43e MK |
249 | arrival/wakeup absolute deadline |
250 | | start time | | |
251 | | | | | |
252 | v v v | |
253 | -----x--------xooooooooooooooooo--------x--------x--- | |
0756f58f | 254 | |<- comp. time ->| |
7bd7f43e MK |
255 | |<------- relative deadline ------>| |
256 | |<-------------- period ------------------->| | |
7a0d1838 | 257 | .fi |
91c98da6 | 258 | .in |
7a0d1838 | 259 | |
9cc1fa25 | 260 | When setting a |
91c98da6 | 261 | .B SCHED_DEADLINE |
9cc1fa25 MK |
262 | policy for a thread using |
263 | .BR sched_setattr (2), | |
264 | one can specify three parameters: | |
265 | .IR Runtime , | |
266 | .IR Deadline , | |
267 | and | |
268 | .IR Period . | |
269 | These parameters do not necessarily correspond to the aforementioned terms: | |
270 | usual practice is to set Runtime to something bigger than the average | |
271 | computation time (or worst-case execution time for hard real-time tasks), | |
272 | Deadline to the relative deadline, and Period to the period of the task. | |
273 | Thus, for | |
274 | .BR SCHED_DEADLINE | |
275 | scheduling, we have: | |
7a0d1838 | 276 | |
91c98da6 | 277 | .in +4n |
7a0d1838 | 278 | .nf |
7bd7f43e MK |
279 | arrival/wakeup absolute deadline |
280 | | start time | | |
281 | | | | | |
282 | v v v | |
283 | -----x--------xooooooooooooooooo--------x--------x--- | |
284 | |<-- Runtime ------->| | |
285 | |<----------- Deadline ----------->| | |
286 | |<-------------- Period ------------------->| | |
7a0d1838 | 287 | .fi |
91c98da6 | 288 | .in |
7a0d1838 | 289 | |
9cc1fa25 MK |
290 | The three deadline-scheduling parameters correspond to the |
291 | .IR sched_runtime , | |
292 | .IR sched_deadline , | |
293 | and | |
294 | .IR sched_period | |
295 | fields of the | |
296 | .I sched_attr | |
297 | structure; see | |
298 | .BR sched_setattr (2). | |
a68beb35 | 299 | These fields express values in nanoseconds. |
9cc1fa25 | 300 | .\" FIXME It looks as though specifying sched_period as 0 means |
47a2bb17 MK |
301 | .\" "make sched_period the same as sched_deadline". |
302 | .\" This needs to be documented. | |
9cc1fa25 MK |
303 | If |
304 | .IR sched_period | |
305 | is specified as 0, then it is made the same as | |
306 | .IR sched_deadline . | |
307 | ||
308 | The kernel requires that: | |
309 | ||
310 | sched_runtime <= sched_deadline <= sched_period | |
311 | ||
312 | .\" See __checkparam_dl in kernel/sched/core.c | |
313 | In addition, under the current implementation, | |
314 | all of the parameter values must be at least 1024 | |
315 | (i.e., just over one microsecond, | |
7bd7f43e | 316 | which is the resolution of the implementation), and less than 2^63. |
9cc1fa25 MK |
317 | If any of these checks fails, |
318 | .BR sched_setattr (2) | |
319 | fails with the error | |
320 | .BR EINVAL . | |
7a0d1838 PZ |
321 | |
322 | The CBS guarantees non-interference between tasks, by throttling | |
9cc1fa25 | 323 | threads that attempt to over-run their specified Runtime. |
7a0d1838 | 324 | |
9cc1fa25 | 325 | To ensure deadline scheduling guarantees, |
0da5e58a | 326 | the kernel must prevent situations where the set of |
91c98da6 | 327 | .B SCHED_DEADLINE |
9cc1fa25 MK |
328 | threads is not feasible (schedulable) within the given constraints. |
329 | The kernel thus performs an admittance test when setting or changing | |
91c98da6 | 330 | .B SCHED_DEADLINE |
9cc1fa25 MK |
331 | policy and attributes. |
332 | This admission test calculates whether the change is feasible; | |
8e8cd193 | 333 | if it is not, |
91c98da6 | 334 | .BR sched_setattr (2) |
9cc1fa25 MK |
335 | fails with the error |
336 | .BR EBUSY . | |
7a0d1838 PZ |
337 | |
338 | For example, it is required (but not necessarily sufficient) for | |
9cc1fa25 MK |
339 | the total utilization to be less than or equal to the total number of |
340 | CPUs available, where, since each thread can maximally run for | |
341 | Runtime per Period, that thread's utilization is its | |
342 | Runtime divided by its Period. | |
7a0d1838 | 343 | |
88e28f78 | 344 | In order to fulfill the guarantees that are made when |
9cc1fa25 MK |
345 | a thread is admitted to the |
346 | .BR SCHED_DEADLINE | |
347 | policy, | |
91c98da6 | 348 | .BR SCHED_DEADLINE |
9cc1fa25 MK |
349 | threads are the highest priority (user controllable) threads in the |
350 | system; if any | |
91c98da6 | 351 | .BR SCHED_DEADLINE |
9cc1fa25 MK |
352 | thread is runnable, |
353 | it will preempt any thread scheduled under one of the other policies. | |
7a0d1838 | 354 | |
9cc1fa25 | 355 | A call to |
91c98da6 | 356 | .BR fork (2) |
9cc1fa25 MK |
357 | by a thread scheduled under the |
358 | .B SCHED_DEADLINE | |
359 | policy will fail with the error | |
360 | .BR EAGAIN , | |
361 | unless the thread has its reset-on-fork flag set (see below). | |
7a0d1838 | 362 | |
91c98da6 MK |
363 | A |
364 | .B SCHED_DEADLINE | |
9cc1fa25 | 365 | thread that calls |
91c98da6 | 366 | .BR sched_yield (2) |
9cc1fa25 MK |
367 | will yield the current job and wait for a new period to begin. |
368 | .\" | |
369 | .\" FIXME Calling sched_getparam() on a SCHED_DEADLINE thread | |
47a2bb17 MK |
370 | .\" fails with EINVAL, but sched_getscheduler() succeeds. |
371 | .\" Is that intended? (Why?) | |
91c98da6 | 372 | .\" |
59c06be3 | 373 | .SS SCHED_OTHER: Default Linux time-sharing scheduling |
b8986eae MK |
374 | \fBSCHED_OTHER\fP can be used at only static priority 0 |
375 | (i.e., threads under real-time policies always have priority over | |
376 | .B SCHED_OTHER | |
377 | processes). | |
59c06be3 MK |
378 | \fBSCHED_OTHER\fP is the standard Linux time-sharing scheduler that is |
379 | intended for all threads that do not require the special | |
380 | real-time mechanisms. | |
f677bcfb | 381 | |
59c06be3 MK |
382 | The thread to run is chosen from the static |
383 | priority 0 list based on a \fIdynamic\fP priority that is determined only | |
384 | inside this list. | |
2be50a32 | 385 | The dynamic priority is based on the nice value (see below) |
927d0dfa | 386 | and is increased for each time quantum the thread is ready to run, |
59c06be3 MK |
387 | but denied to run by the scheduler. |
388 | This ensures fair progress among all \fBSCHED_OTHER\fP threads. | |
389 | .\" | |
45fcd0e2 | 390 | .SS The nice value |
bcbb240c | 391 | The nice value is an attribute |
d145138e MK |
392 | that can be used to influence the CPU scheduler to |
393 | favor or disfavor a process in scheduling decisions. | |
394 | It affects the scheduling of | |
395 | .BR SCHED_OTHER | |
396 | and | |
397 | .BR SCHED_BATCH | |
bcbb240c | 398 | (see below) processes. |
2be50a32 MK |
399 | The nice value can be modified using |
400 | .BR nice (2), | |
401 | .BR setpriority (2), | |
402 | or | |
403 | .BR sched_setattr (2). | |
d145138e | 404 | |
bcbb240c MK |
405 | According to POSIX.1, the nice value is a per-process attribute; |
406 | that is, the threads in a process should share a nice value. | |
407 | However, on Linux, the nice value is a per-thread attribute: | |
408 | different threads in the same process may have different nice values. | |
409 | ||
45fcd0e2 MK |
410 | The range of the nice value |
411 | varies across UNIX systems. | |
412 | On modern Linux, the range is \-20 (high priority) to +19 (low priority). | |
413 | On some other systems, the range is \-20..20. | |
414 | Very early Linux kernels (Before Linux 2.0) had the range \-infinity..15. | |
415 | .\" Linux before 1.3.36 had \-infinity..15. | |
416 | .\" Since kernel 1.3.43, Linux has the range \-20..19. | |
417 | ||
418 | The degree to which the nice value affects the relative scheduling of | |
419 | .BR SCHED_OTHER | |
420 | processes likewise varies across UNIX systems and | |
421 | across Linux kernel versions. | |
422 | ||
423 | With the advent of the CFS scheduler in kernel 2.6.23, | |
424 | Linux adopted an algorithm that causes | |
425 | relative differences in nice values to have a much stronger effect. | |
115366c6 MK |
426 | In the current implementation, each unit of difference in the |
427 | nice values of two processes results in a factor of 1.25 | |
428 | in the degree to which the scheduler favors the higher priority process. | |
45fcd0e2 MK |
429 | This causes very low nice values (+19) to truly provide little CPU |
430 | to a process whenever there is any other | |
431 | higher priority load on the system, | |
432 | and makes high nice values (\-20) deliver most of the CPU to applications | |
433 | that require it (e.g., some audio applications). | |
50e12810 MK |
434 | |
435 | On Linux, the | |
436 | .BR RLIMIT_NICE | |
437 | resource limit can be used to define a limit to which | |
438 | an unprivileged process's nice value can be raised; see | |
439 | .BR setrlimit (2) | |
440 | for details. | |
45fcd0e2 | 441 | .\" |
e92070f8 MK |
442 | .SS The nice value and group scheduling |
443 | When scheduling non-real-time processes (i.e., those scheduled under the | |
444 | .BR SCHED_OTHER , | |
445 | .BR SCHED_BATCH , | |
446 | and | |
447 | .BR SCHED_IDLE | |
448 | policies), the CFS scheduler employs a technique known as "group scheduling", | |
449 | if the kernel was configured with the | |
450 | .BR CONFIG_GROUP_SCHED | |
451 | option (which is typical). | |
452 | Scheduling groups (also known as task groups) | |
453 | are formed in the following circumstances: | |
454 | .IP * 3 | |
455 | All of the processes in a non-root CPU cgroup form a scheduling group. | |
456 | Thus, each non-root CPU cgroup is a scheduling group. | |
457 | .IP * | |
458 | All of the processes that are (implicitly) placed in an autogroup | |
459 | (i.e., the same session, as created by | |
460 | .BR setsid (2)) | |
461 | form a scheduling group. | |
462 | Each new autogroup is thus a separate scheduling group. | |
463 | .IP * | |
464 | If autogrouping is disabled, then all processes in the root CPU cgroup | |
465 | form a scheduling group (sometimes called the "root task group"). | |
466 | .PP | |
467 | Under group scheduling, | |
468 | the nice value has an effect for scheduling decisions | |
469 | .IR "only relative to other threads in the same group" . | |
470 | This has some surprising consequences in terms of the traditional semantics | |
471 | of the nice value on UNIX systems. | |
472 | In particular, if autogrouping | |
473 | is enabled (which is the default), then employing | |
474 | .BR setpriority (2) | |
475 | or | |
476 | .BR nice (1) | |
477 | on a process has an effect only for scheduling relative | |
478 | to other processes executed in the same session | |
479 | (typically: the same terminal window). | |
480 | Conversely, for two process that are (for example) the sole CPU-bound processes | |
481 | in different sessions (different terminal windows), | |
482 | .IR "modifying the nice value of the process in one of the sessions" | |
483 | .IR "has no effect" | |
484 | in terms of the scheduler's decisions relative to the | |
485 | process in the other session. | |
486 | .\" More succinctly: the nice(1) command is nearly a no-op since Linux 2.6.38. | |
487 | .\" | |
59c06be3 MK |
488 | .SS SCHED_BATCH: Scheduling batch processes |
489 | (Since Linux 2.6.16.) | |
490 | \fBSCHED_BATCH\fP can be used only at static priority 0. | |
491 | This policy is similar to \fBSCHED_OTHER\fP in that it schedules | |
492 | the thread according to its dynamic priority | |
493 | (based on the nice value). | |
494 | The difference is that this policy | |
495 | will cause the scheduler to always assume | |
496 | that the thread is CPU-intensive. | |
497 | Consequently, the scheduler will apply a small scheduling | |
a1fa36af | 498 | penalty with respect to wakeup behavior, |
59c06be3 MK |
499 | so that this thread is mildly disfavored in scheduling decisions. |
500 | ||
501 | .\" The following paragraph is drawn largely from the text that | |
502 | .\" accompanied Ingo Molnar's patch for the implementation of | |
503 | .\" SCHED_BATCH. | |
504 | .\" commit b0a9499c3dd50d333e2aedb7e894873c58da3785 | |
505 | This policy is useful for workloads that are noninteractive, | |
506 | but do not want to lower their nice value, | |
507 | and for workloads that want a deterministic scheduling policy without | |
508 | interactivity causing extra preemptions (between the workload's tasks). | |
509 | .\" | |
510 | .SS SCHED_IDLE: Scheduling very low priority jobs | |
511 | (Since Linux 2.6.23.) | |
512 | \fBSCHED_IDLE\fP can be used only at static priority 0; | |
513 | the process nice value has no influence for this policy. | |
514 | ||
515 | This policy is intended for running jobs at extremely low | |
516 | priority (lower even than a +19 nice value with the | |
517 | .B SCHED_OTHER | |
518 | or | |
519 | .B SCHED_BATCH | |
520 | policies). | |
521 | .\" | |
522 | .SS Resetting scheduling policy for child processes | |
005eaa8f MK |
523 | Each thread has a reset-on-fork scheduling flag. |
524 | When this flag is set, children created by | |
525 | .BR fork (2) | |
526 | do not inherit privileged scheduling policies. | |
527 | The reset-on-fork flag can be set by either: | |
528 | .IP * 3 | |
529 | ORing the | |
59c06be3 | 530 | .B SCHED_RESET_ON_FORK |
005eaa8f | 531 | flag into the |
59c06be3 | 532 | .I policy |
005eaa8f MK |
533 | argument when calling |
534 | .BR sched_setscheduler (2) | |
535 | (since Linux 2.6.32); | |
536 | or | |
537 | .IP * | |
538 | specifying the | |
539 | .B SCHED_FLAG_RESET_ON_FORK | |
540 | flag in | |
541 | .IR attr.sched_flags | |
59c06be3 | 542 | when calling |
005eaa8f MK |
543 | .BR sched_setattr (2). |
544 | .PP | |
545 | Note that the constants used with these two APIs have different names. | |
546 | The state of the reset-on-fork flag can analogously be retrieved using | |
547 | .BR sched_getscheduler (2) | |
548 | and | |
549 | .BR sched_getattr (2). | |
550 | ||
551 | The reset-on-fork feature is intended for media-playback applications, | |
59c06be3 MK |
552 | and can be used to prevent applications evading the |
553 | .BR RLIMIT_RTTIME | |
554 | resource limit (see | |
555 | .BR getrlimit (2)) | |
556 | by creating multiple child processes. | |
557 | ||
005eaa8f | 558 | More precisely, if the reset-on-fork flag is set, |
59c06be3 MK |
559 | the following rules apply for subsequently created children: |
560 | .IP * 3 | |
561 | If the calling thread has a scheduling policy of | |
562 | .B SCHED_FIFO | |
563 | or | |
564 | .BR SCHED_RR , | |
565 | the policy is reset to | |
566 | .BR SCHED_OTHER | |
567 | in child processes. | |
568 | .IP * | |
569 | If the calling process has a negative nice value, | |
570 | the nice value is reset to zero in child processes. | |
571 | .PP | |
005eaa8f | 572 | After the reset-on-fork flag has been enabled, |
59c06be3 MK |
573 | it can be reset only if the thread has the |
574 | .BR CAP_SYS_NICE | |
575 | capability. | |
576 | This flag is disabled in child processes created by | |
577 | .BR fork (2). | |
59c06be3 MK |
578 | .\" |
579 | .SS Privileges and resource limits | |
580 | In Linux kernels before 2.6.12, only privileged | |
581 | .RB ( CAP_SYS_NICE ) | |
582 | threads can set a nonzero static priority (i.e., set a real-time | |
583 | scheduling policy). | |
584 | The only change that an unprivileged thread can make is to set the | |
585 | .B SCHED_OTHER | |
759e1210 | 586 | policy, and this can be done only if the effective user ID of the caller |
59c06be3 MK |
587 | matches the real or effective user ID of the target thread |
588 | (i.e., the thread specified by | |
589 | .IR pid ) | |
590 | whose policy is being changed. | |
591 | ||
9cc1fa25 MK |
592 | A thread must be privileged |
593 | .RB ( CAP_SYS_NICE ) | |
0da5e58a | 594 | in order to set or modify a |
9cc1fa25 MK |
595 | .BR SCHED_DEADLINE |
596 | policy. | |
597 | ||
59c06be3 MK |
598 | Since Linux 2.6.12, the |
599 | .B RLIMIT_RTPRIO | |
600 | resource limit defines a ceiling on an unprivileged thread's | |
601 | static priority for the | |
602 | .B SCHED_RR | |
603 | and | |
604 | .B SCHED_FIFO | |
605 | policies. | |
606 | The rules for changing scheduling policy and priority are as follows: | |
607 | .IP * 3 | |
608 | If an unprivileged thread has a nonzero | |
609 | .B RLIMIT_RTPRIO | |
610 | soft limit, then it can change its scheduling policy and priority, | |
611 | subject to the restriction that the priority cannot be set to a | |
612 | value higher than the maximum of its current priority and its | |
613 | .B RLIMIT_RTPRIO | |
614 | soft limit. | |
615 | .IP * | |
616 | If the | |
617 | .B RLIMIT_RTPRIO | |
618 | soft limit is 0, then the only permitted changes are to lower the priority, | |
619 | or to switch to a non-real-time policy. | |
620 | .IP * | |
621 | Subject to the same rules, | |
622 | another unprivileged thread can also make these changes, | |
623 | as long as the effective user ID of the thread making the change | |
624 | matches the real or effective user ID of the target thread. | |
625 | .IP * | |
626 | Special rules apply for the | |
f7a858b4 MK |
627 | .BR SCHED_IDLE |
628 | policy. | |
59c06be3 MK |
629 | In Linux kernels before 2.6.39, |
630 | an unprivileged thread operating under this policy cannot | |
631 | change its policy, regardless of the value of its | |
632 | .BR RLIMIT_RTPRIO | |
633 | resource limit. | |
634 | In Linux kernels since 2.6.39, | |
635 | .\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8 | |
636 | an unprivileged thread can switch to either the | |
637 | .BR SCHED_BATCH | |
638 | or the | |
85b6211a | 639 | .BR SCHED_OTHER |
59c06be3 MK |
640 | policy so long as its nice value falls within the range permitted by its |
641 | .BR RLIMIT_NICE | |
642 | resource limit (see | |
643 | .BR getrlimit (2)). | |
644 | .PP | |
645 | Privileged | |
646 | .RB ( CAP_SYS_NICE ) | |
647 | threads ignore the | |
648 | .B RLIMIT_RTPRIO | |
649 | limit; as with older kernels, | |
650 | they can make arbitrary changes to scheduling policy and priority. | |
651 | See | |
652 | .BR getrlimit (2) | |
653 | for further information on | |
654 | .BR RLIMIT_RTPRIO . | |
0c055c75 MK |
655 | .SS Limiting the CPU usage of real-time and deadline processes |
656 | A nonblocking infinite loop in a thread scheduled under the | |
657 | .BR SCHED_FIFO , | |
658 | .BR SCHED_RR , | |
659 | or | |
660 | .BR SCHED_DEADLINE | |
df312a96 MK |
661 | policy can potentially block all other threads from accessing |
662 | the CPU forever. | |
0c055c75 MK |
663 | Prior to Linux 2.6.25, the only way of preventing a runaway real-time |
664 | process from freezing the system was to run (at the console) | |
665 | a shell scheduled under a higher static priority than the tested application. | |
666 | This allows an emergency kill of tested | |
667 | real-time applications that do not block or terminate as expected. | |
668 | ||
669 | Since Linux 2.6.25, there are other techniques for dealing with runaway | |
670 | real-time and deadline processes. | |
671 | One of these is to use the | |
672 | .BR RLIMIT_RTTIME | |
673 | resource limit to set a ceiling on the CPU time that | |
674 | a real-time process may consume. | |
675 | See | |
676 | .BR getrlimit (2) | |
677 | for details. | |
678 | ||
679 | Since version 2.6.25, Linux also provides two | |
680 | .I /proc | |
681 | files that can be used to reserve a certain amount of CPU time | |
682 | to be used by non-real-time processes. | |
0b1ce085 | 683 | Reserving CPU time in this fashion allows some CPU time to be |
0c055c75 MK |
684 | allocated to (say) a root shell that can be used to kill a runaway process. |
685 | Both of these files specify time values in microseconds: | |
686 | .TP | |
687 | .IR /proc/sys/kernel/sched_rt_period_us | |
688 | This file specifies a scheduling period that is equivalent to | |
689 | 100% CPU bandwidth. | |
690 | The value in this file can range from 1 to | |
691 | .BR INT_MAX , | |
692 | giving an operating range of 1 microsecond to around 35 minutes. | |
693 | The default value in this file is 1,000,000 (1 second). | |
694 | .TP | |
695 | .IR /proc/sys/kernel/sched_rt_runtime_us | |
696 | The value in this file specifies how much of the "period" time | |
697 | can be used by all real-time and deadline scheduled processes | |
698 | on the system. | |
699 | The value in this file can range from \-1 to | |
700 | .BR INT_MAX \-1. | |
701 | Specifying \-1 makes the runtime the same as the period; | |
702 | that is, no CPU time is set aside for non-real-time processes | |
703 | (which was the Linux behavior before kernel 2.6.25). | |
704 | The default value in this file is 950,000 (0.95 seconds), | |
705 | meaning that 5% of the CPU time is reserved for processes that | |
706 | don't run under a real-time or deadline scheduling policy. | |
707 | .PP | |
59c06be3 | 708 | .SS Response time |
1154a064 | 709 | A blocked high priority thread waiting for I/O has a certain |
59c06be3 MK |
710 | response time before it is scheduled again. |
711 | The device driver writer | |
712 | can greatly reduce this response time by using a "slow interrupt" | |
713 | interrupt handler. | |
714 | .\" as described in | |
715 | .\" .BR request_irq (9). | |
716 | .SS Miscellaneous | |
717 | Child processes inherit the scheduling policy and parameters across a | |
718 | .BR fork (2). | |
719 | The scheduling policy and parameters are preserved across | |
720 | .BR execve (2). | |
721 | ||
722 | Memory locking is usually needed for real-time processes to avoid | |
723 | paging delays; this can be done with | |
724 | .BR mlock (2) | |
725 | or | |
726 | .BR mlockall (2). | |
ed520068 MK |
727 | .\" |
728 | .SS The autogroup feature | |
729 | .\" commit 5091faa449ee0b7d73bc296a93bca9540fc51d0a | |
730 | Since Linux 2.6.38, | |
731 | the kernel provides a feature known as autogrouping to improve interactive | |
ee1f3c18 | 732 | desktop performance in the face of multiprocess, CPU-intensive |
ed520068 MK |
733 | workloads such as building the Linux kernel with large numbers of |
734 | parallel build processes (i.e., the | |
735 | .BR make (1) | |
736 | .BR \-j | |
737 | flag). | |
738 | ||
739 | This feature operates in conjunction with the | |
740 | CFS scheduler and requires a kernel that is configured with | |
741 | .BR CONFIG_SCHED_AUTOGROUP . | |
742 | On a running system, this feature is enabled or disabled via the file | |
743 | .IR /proc/sys/kernel/sched_autogroup_enabled ; | |
744 | a value of 0 disables the feature, while a value of 1 enables it. | |
745 | The default value in this file is 1, unless the kernel was booted with the | |
746 | .IR noautogroup | |
747 | parameter. | |
748 | ||
ee1f3c18 MK |
749 | A new autogroup is created created when a new session is created via |
750 | .BR setsid (2); | |
751 | this happens, for example, when a new terminal window is started. | |
752 | A new process created by | |
753 | .BR fork (2) | |
754 | inherits its parent's autogroup membership. | |
755 | Thus, all of the processes in a session are members of the same autogroup. | |
756 | An autogroup is automatically destroyed when the last process | |
ed520068 MK |
757 | in the group terminates. |
758 | ||
ee1f3c18 MK |
759 | When autogrouping is enabled, all of the members of an autogroup |
760 | are placed in the same kernel scheduler "task group". | |
761 | The CFS scheduler employs an algorithm that equalizes the | |
762 | distribution of CPU cycles across task groups. | |
763 | The benefits of this for interactive desktop performance | |
764 | can be described via the following example. | |
765 | ||
626dca36 MK |
766 | Suppose that there are two autogroups competing for the same CPU |
767 | (i.e., presume either a single CPU system or the use of | |
768 | .BR taskset (1) | |
769 | to confine all the processes to the same CPU on an SMP system). | |
ee1f3c18 MK |
770 | The first group contains ten CPU-bound processes from |
771 | a kernel build started with | |
772 | .IR "make\ \-j10" . | |
773 | The other contains a single CPU-bound process: a video player. | |
774 | The effect of autogrouping is that the two groups will | |
775 | each receive half of the CPU cycles. | |
776 | That is, the video player will receive 50% of the CPU cycles, | |
c11d0670 | 777 | rather than just 9% of the cycles, |
ee1f3c18 | 778 | which would likely lead to degraded video playback. |
626dca36 MK |
779 | The situation on an SMP system is more complex, |
780 | .\" Mike Galbraith, 25 Nov 2016: | |
781 | .\" I'd say something more wishy-washy here, like cycles are | |
782 | .\" distributed fairly across groups and leave it at that, as your | |
783 | .\" detailed example is incorrect due to SMP fairness (which I don't | |
784 | .\" like much because [very unlikely] worst case scenario | |
785 | .\" renders a box sized group incapable of utilizing more that | |
786 | .\" a single CPU total). For example, if a group of NR_CPUS | |
787 | .\" size competes with a singleton, load balancing will try to give | |
788 | .\" the singleton a full CPU of its very own. If groups intersect for | |
789 | .\" whatever reason on say my quad lappy, distribution is 80/20 in | |
790 | .\" favor of the singleton. | |
791 | but the general effect is the same: | |
792 | the scheduler distributes CPU cycles across task groups such that | |
ee1f3c18 | 793 | an autogroup that contains a large number of CPU-bound processes |
1dd83d2e | 794 | does not end up hogging CPU cycles at the expense of the other |
ee1f3c18 MK |
795 | jobs on the system. |
796 | ||
797 | A process's autogroup (task group) membership can be viewed via the file | |
ed520068 MK |
798 | .IR /proc/[pid]/autogroup : |
799 | ||
800 | .nf | |
801 | .in +4n | |
802 | $ \fBcat /proc/1/autogroup\fP | |
803 | /autogroup-1 nice 0 | |
804 | .in | |
805 | .fi | |
806 | ||
807 | This file can also be used to modify the CPU bandwidth allocated | |
ee1f3c18 | 808 | to an autogroup. |
ed520068 | 809 | This is done by writing a number in the "nice" range to the file |
ee1f3c18 | 810 | to set the autogroup's nice value. |
58627ec0 MK |
811 | The allowed range is from +19 (low priority) to \-20 (high priority). |
812 | (Writing values outside of this range causes | |
813 | .BR write (2) | |
814 | to fail with the error | |
815 | .BR EINVAL .) | |
816 | The setting has the same effect as modifying the nice level via | |
ee1f3c18 | 817 | .BR getpriority (2). |
576b74ee MK |
818 | (For a discussion of the nice value, see |
819 | .BR getpriority (2).) | |
ee1f3c18 | 820 | .\" FIXME . |
576b74ee MK |
821 | .\" Because of a bug introduced in Linux 4.7 |
822 | .\" (commit 2159197d66770ec01f75c93fb11dc66df81fd45b made changes | |
823 | .\" that exposed the fact that autogroup didn't call scale_load()), | |
824 | .\" it happened that *all* values in this range caused a task group | |
825 | .\" to be further disfavored by the scheduler, with \-20 resulting | |
826 | .\" in the scheduler mildy disfavoring the task group and +19 greatly | |
827 | .\" disfavoring it. | |
ed520068 | 828 | .\" |
576b74ee MK |
829 | .\" A patch was posted on 23 Nov 2016 |
830 | .\" ("sched/autogroup: Fix 64bit kernel nice adjustment"; | |
831 | .\" check later to see in which kernel version it lands. | |
ee1f3c18 MK |
832 | .\" |
833 | .\" FIXME How do the nice value of a process and the nice value of | |
834 | .\" an autogroup interact? Which has priority? | |
45922aa8 MK |
835 | .\" |
836 | .\" It *appears* that the autogroup nice value is used for CPU distribution | |
837 | .\" between task groups, and that the process nice value has no effect there. | |
838 | .\" (I.e., suppose two autogroups each contain a CPU-bound process, | |
839 | .\" with one process having nice==0 and the other having nice==19. | |
840 | .\" It appears that they each get 50% of the CPU.) | |
841 | .\" It appears that the process nice value has effect only with respect to | |
842 | .\" scheduling relative to other processes in the *same* autogroup. | |
843 | .\" Is this correct? | |
ed520068 | 844 | |
ed520068 MK |
845 | The use of the |
846 | .BR cgroups (7) | |
e9c1649a MK |
847 | CPU controller to place processes in cgroups other than the |
848 | root CPU cgroup overrides the effect of autogrouping. | |
ee1f3c18 MK |
849 | |
850 | The autogroup feature does not group processes | |
851 | that are scheduled under a real-time and deadline policies. | |
852 | Those processes are scheduled according to the rules described earlier. | |
59c06be3 | 853 | .SH NOTES |
1f7fb9c0 MK |
854 | The |
855 | .BR cgroups (7) | |
856 | CPU controller can be used to limit the CPU consumption of | |
857 | groups of processes. | |
59c06be3 MK |
858 | .PP |
859 | Originally, Standard Linux was intended as a general-purpose operating | |
860 | system being able to handle background processes, interactive | |
861 | applications, and less demanding real-time applications (applications that | |
862 | need to usually meet timing deadlines). | |
863 | Although the Linux kernel 2.6 | |
864 | allowed for kernel preemption and the newly introduced O(1) scheduler | |
865 | ensures that the time needed to schedule is fixed and deterministic | |
866 | irrespective of the number of active tasks, true real-time computing | |
867 | was not possible up to kernel version 2.6.17. | |
868 | .SS Real-time features in the mainline Linux kernel | |
869 | .\" FIXME . Probably this text will need some minor tweaking | |
84dd1325 | 870 | .\" ask Carsten Emde about this. |
6ad8b4d0 | 871 | Since kernel version 2.6.18, Linux is gradually |
59c06be3 MK |
872 | becoming equipped with real-time capabilities, |
873 | most of which are derived from the former | |
874 | .I realtime-preempt | |
94875d76 | 875 | patch set. |
59c06be3 | 876 | Until the patches have been completely merged into the |
84dd1325 | 877 | mainline kernel, |
59c06be3 MK |
878 | they must be installed to achieve the best real-time performance. |
879 | These patches are named: | |
880 | .in +4n | |
881 | .nf | |
882 | ||
883 | patch-\fIkernelversion\fP-rt\fIpatchversion\fP | |
884 | .fi | |
885 | .in | |
886 | .PP | |
887 | and can be downloaded from | |
888 | .UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/ | |
889 | .UE . | |
890 | ||
891 | Without the patches and prior to their full inclusion into the mainline | |
892 | kernel, the kernel configuration offers only the three preemption classes | |
893 | .BR CONFIG_PREEMPT_NONE , | |
894 | .BR CONFIG_PREEMPT_VOLUNTARY , | |
895 | and | |
896 | .B CONFIG_PREEMPT_DESKTOP | |
897 | which respectively provide no, some, and considerable | |
898 | reduction of the worst-case scheduling latency. | |
899 | ||
900 | With the patches applied or after their full inclusion into the mainline | |
901 | kernel, the additional configuration item | |
902 | .B CONFIG_PREEMPT_RT | |
903 | becomes available. | |
904 | If this is selected, Linux is transformed into a regular | |
905 | real-time operating system. | |
759e1210 | 906 | The FIFO and RR scheduling policies are then used to run a thread |
59c06be3 | 907 | with true real-time priority and a minimum worst-case scheduling latency. |
59c06be3 MK |
908 | .SH SEE ALSO |
909 | .ad l | |
910 | .nh | |
911 | .BR chrt (1), | |
f19db853 | 912 | .BR taskset (1), |
59c06be3 MK |
913 | .BR getpriority (2), |
914 | .BR mlock (2), | |
915 | .BR mlockall (2), | |
916 | .BR munlock (2), | |
917 | .BR munlockall (2), | |
918 | .BR nice (2), | |
919 | .BR sched_get_priority_max (2), | |
920 | .BR sched_get_priority_min (2), | |
921 | .BR sched_getaffinity (2), | |
922 | .BR sched_getparam (2), | |
584c8ee0 | 923 | .BR sched_getscheduler (2), |
59c06be3 MK |
924 | .BR sched_rr_get_interval (2), |
925 | .BR sched_setaffinity (2), | |
926 | .BR sched_setparam (2), | |
584c8ee0 | 927 | .BR sched_setscheduler (2), |
59c06be3 MK |
928 | .BR sched_yield (2), |
929 | .BR setpriority (2), | |
720a5280 MK |
930 | .BR pthread_getaffinity_np (3), |
931 | .BR pthread_setaffinity_np (3), | |
932 | .BR sched_getcpu (3), | |
59c06be3 MK |
933 | .BR capabilities (7), |
934 | .BR cpuset (7) | |
935 | .ad | |
936 | .PP | |
937 | .I Programming for the real world \- POSIX.4 | |
938 | by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0. | |
939 | .PP | |
b963d0e3 MK |
940 | The Linux kernel source files |
941 | .IR Documentation/scheduler/sched-deadline.txt , | |
942 | .IR Documentation/scheduler/sched-rt-group.txt , | |
458689ed | 943 | .IR Documentation/scheduler/sched-design-CFS.txt , |
b963d0e3 | 944 | and |
d630434e | 945 | .IR Documentation/scheduler/sched-nice-design.txt |